1. Field of the Invention
The Department of Energy has estimated that about 1.8 trillion tons of now unrecoverable domestic coal could be exploited to produce gaseous and/or liquid fuels via underground coal gasification (UCG). This 1.8 trillion ton resource involves coal that is too deep, too steeply dipping, or of marginal quality for economic recovery by mining. Accordingly, UCG has enormous potential for providing a long-term gaseous and/or liquid fuel supply and may be the only economic method of recovering existing unminable domestic coal resources.
The chemistry of underground, or in-situ, coal gasification is similar to that observed in the surface gasification of coal, a process developed by Lurgi and others. The coal is reacted in the underground coal seam using an oxidant such as oxygen and steam to produce hydrogen, carbon monoxide, methane, carbon dioxide, and small concentrations of some other compounds. These gases are piped to a surface facility where the reactive species are converted to synthetic natural gas (SNG) (methane) and/or liquid fuels such as methanol, gasoline, or diesel fuel. Partial oxidation of the coal supplies the heat necessary to develop temperatures of 1800.degree.-2200.degree. F. required to drive the endothermic gasification reactions.
To date, application of UCG to recover coal resources on a semicommercial or commercial scale has been practiced only in the U.S.S.R., the study of which began as early as 1927. This work continued on air-blown underground coal UCG with the intent of producing low-Btu gas for industrial fuel and generation of electricity.
As in surface gasification, the use of steam and oxygen is a prerequisite for economically producing SNG and/or liquid fuels. Tests conducted in this country have demonstrated the feasibility of oxygen-blown gasification of coal in both flat-lying and steeply dipping seams. The gas gathered in such a system consists principally of hydrogen, carbon monoxide, carbon dioxide, water vapor, and methane. Minor constituents include hydrogen sulfide, ammonia, and entrained particles carried by the gas stream. This gas mixture is treated in a surface plant to remove and separate the methane, particulates, carbon dioxide, hydrogen sulfide, ammonia, and tars producing a gas consisting primarily of carbon monoxide, and hydrogen. This latter gas, sometimes called synthesis gas, can be piped to a central gas processing plant for conversion to SNG or liquid fuels. Processing steps include: (1) quench and scrubbing, (2) compression and shift, (3) acid gas removal and sulfur recovery, and (4) conversion.
2. Related Art
The Lurgi process has been mentioned. The drawing of a typical gasifier for this process is shown on page 206 of "Synthetic Fuels Data Handbook," compiled by Dr. Thomas A. Hendrickson, copyright 1975, by Cameron Engineers, Inc. The next page of this publication discloses that El Paso Natural Gas Company has proposed the use of Lurgi gasifiers for the Burnham Coal Gasification Complex to be located near Farmington, New Mexico. In this plant, gasifiers are shown in two applications. One group of gasifiers is oxygen blown and produces a relatively high methane content gas which is upgraded to pipeline gas quality. In the second application, air-blown Lurgi gasifiers produce low-Btu gas for in-plant use to generate process steam and electric power.
The coal gasification steps produce, as the output of the Lurgi gasifier, either a medium-Btu or a low-Btu content stream. The terms "low," "medium," and "high," are not specific defined limits. However, low Btu is generally considered to contain less than 200 Btu/SCF, medium-Btu gas would contain 200 to 400 Btu/SCF. All these values are based on dry gas.